Process Media Process Steps

The purification and recovery of a biological target molecule out of a tissue, cell or fermentation broth - the so called downstream processing - usually requires a combination of separation technologies and involves more than one chromatographic unit operation.

Downstream processing operations are often divided into three groups describing the continuous improvements in purity and concentration of the product: Capture, Intermediate Purification and Polishing. In parallel to the dimensions of the chromatography columns applied also the particle sizes of the chromatographic resins decrease along the progress of the purification train.

Quality control methods applied to confirm the purity of the final product also involve liquid chromatography. A variety of TSKgel (U)HPLC columns are available for QC analysis of biologics.

The capture step is the first process step just downstream of the harvested feedstock. Its primary function is to "capture" the target from solution and separate it from the most dominant impurities. The target is then eluted into a significantly smaller volume of buffer for further downstream processing. A resin is selected which has the best combination of dynamic binding capacity, mass recovery, and retention of the target's biological activity.

Column dimensions and resin particle sizes tend to be larger in capture steps. The feedstock viscosity, the amount of particulate, and the operational pressure limits of the chromatographic hardware used, determine the most appropriate particle size for a capture step. The more viscous and particulate laden the feedstock, the larger the particle needed to produce a linear velocity suitable for the process throughput objectives.

The partially purified feedstock from the capture step will have a higher target concentration and relatively less complex impurity profile (also at higher concentrations). The iintermediate purification step uses chromatography's resolving power to separate the target away from other impurities of the feedstock. Because of this, and the improved fluid flow properties, resin particle sizes of 65 µm and smaller are used. In many cases, more than one intermediate purification column may be necessary to achieve the desired degree of product purity.

The final column in a production train is often referred to as the polishing step. At this point the feedstock contains a relatively high concentration of the target molecule with some select impurities or aggregates. The residual impurities however may be very closely related to the target and may be dimer, trimer, or isoforms of the target. Resins with high selectivity and highly efficient smaller particle sizes of 35 µm and smaller are typically used.